Hydroxylamine and hydroxyl-ammonium salts are used in connection with the manufacture of caprolactam. Cyclohexanone and the hydroxylamine or its salt, are reacted to form cyclohexanone oxime; the cyclohexanone oxime is further reacted to form the caprolactam.
Hydroxylamine, and salts thereof, can be prepared by reducing nitrogen monoxide, nitric acid, or a nitrate by reducing the aforementioned compounds. A well known prior art method for reducing nitrogen monoxide, nitric acid or a nitrate to the hydroxylammonium salt of hydroxylamine is by treating those compounds with hydrogen and a hydrogenation catalyst, e.g., palladium, platinum or mixtures thereof on a support, in an acidic medium. U.S. Pat. No. 3,767,758 and British Patent Specification No. 772,693 disclose such processes, both of said references being incorporated herein by reference.
In such a catalytic hydrogenation of nitrogen oxide compounds, the reaction mixture must be treated to remove catalyst particles therefrom. Normally, the method of separation is by filtration. The reaction medium resulting from the reduction of those nitrogen oxide compounds is in the form of a suspension-solution. It has been discovered that filtration of the suspension-solution reaction mixture is difficult, since filtration resistance increases with time during the filtration of that reaction medium. The filtration resistance values, measured as drainage time through the filter, may rise to very high values. This filtration resistance is detrimental to procedures of preparation of the hydroxyl-ammonium salt, and to its isolation. The rise of filtration resistance appears to be caused by the presence of small amounts of impurities with a peptizing action; such impurities include, for example, succinic acid, hydroxylcarboxylic acids, amines, and toluene. These organic impurities can be present in the reaction mixture when the hydroxylammonium salt-containing reaction mixture is further reacted with a ketone to form an oxime and, after removal of the oxime from the reaction mixture, the remaining impure reaction medium is re-used in the preparation of a further amount of hydroxylammonium salt.